It is known that bifidobacteria are useful inhabitants of the human intestine and it is known that as they are colonized there, these bacteria produce lactic acid, acetic acid and formic acid to lower the pH in the intestinal tract and thereby tend to preclude local settlement of pathogenic organisms. Compounds having activity to encourage the growth and proliferation of these useful bifidobacteria have been utilized as additives incorporated into various foods such as evaporated milk, beverages and so on.
Much research has been undertaken into growth promoting factors for bifidobacteria (hereinafter referred to as bifidus factors) and as such factors, there have been reported lactulose, fructo-oligosaccharides, galacto-oligosaccharides of the general formula Gal--(Gal).sub.n --Glc (wherein Gal is a galactose residue, Galc is a glucose residue, and n is a whole number of 1 to 4), carrot extract, N-acetyllactosamine, and so on.
For the production of a galacto-oligosaccharide from lactose using an enzymatic or microbial technique, there are known a process which employs .beta.-galactosidase produced by Aspergillus oryzae (Japanese Patent Publication No.20266/83; corresponding U.S. Pat. No. 4,435,389), a process which comprises growing a strain of Bacillus sp. in a lactose-containing medium and harvesting the bifidus factor from the culture broth (Japanese Patent Application (OPI) No. 115796/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), a process which comprises growing a yeast of the genus Cryptococcus in a lactose-containing medium and harvesting a galacto-oligosaccharide from the broth (Japanese Patent Application (OPI) No. 251896/85), a process in which a galacto-oligosaccharide is produced using the lactase derived from Saccharomyces fragilis (Agricultural and Food Chemistry 5, 130 k, (1957)), a process in which a strain of Sporobolomyces singularis is cultured in a lactose-containing medium to produce a galacto-oligosaccharide in the broth (Canadian Journal of Chemistry 42, 1341, (1964)), a process which comprises growing a strain of Penicillium chrysogenum in a lactose-containing medium to produce a galacto-oligosaccharide in the broth (Tetrahedron 9, 125, (1960)), a process in which a galacto-oligosaccharide is produced by using a .beta.-galactosidase of Lactobacillus origin (Journal of Dairy Science 64, 185, (1981)), and a process which employs the .beta.-galactosidase derived from Bacillus circulans to produce a galacto-oligosaccharide (Agricultural Biological Chemistry 48, 3053, (1984)), among others.
The foregoing processes for producing bifidus factors from lactose may be roughly classified into those employing enzymes extracted from the microbial cells and those in which a certain strain of microorganism is cultured in a lactose-containing medium and the resulting galacto-oligosaccharide is harvested from the culture broth. Of these two groups of processes, the processes employing the enzymes extracted from microbial cells of course call for the extraction of enzymes which is labor- and time-consuming. Moreover, these processes entail hydrolysis of lactose as well as of the desired galacto-oligosaccharide and the consequent accumulation of glucose and galactose as by-products of the reaction, thus causing a waste of the starting material lactose and leading to a reduced yield of the galacto-oligosaccharide.
On the other hand, the processes involving the production of galacto-oligosaccharides in culture media are disadvantageous in that because the cells grow and multiply, other microbial secretions are also accumulated in the media and interfere with the separation and purification of galacto-oligosaccharides and also in that the desired galacto-oligosaccharides must be separated from the materials incorporated in the medium, namely the nitrogen sources, vitamins, trace elements and so on which are necessary or useful for the growth of bacteria.